1. Field of the Invention
The present invention relates, in general, to a mesoporous molecular sieve substance and methods for preparing the same. More particularly, this invention relates to a noncrystalline mesoporous molecular sieve substance, superior over the M41S series of Mobil Co. in thermal stability, hydrothermal stability and structured in such a manner that channels of uniform diameter are randomly arranged in a three-dimensional way so as to allow the ready diffusion of reactants therein. Also, the present invention is concerned with methods for preparing the noncrystalline mesoporous molecular sieve substance.
2. Description of the Prior Art
Among solid substances known thus far, those having uniform channels, such as zeolites of porous crystalline aluminum silicate and of porous crystalline aluminum phosphates (AlPO.sub.4) are defined as molecular sieves, because they selectively adsorb molecules smaller than the size of the channel entrance or they allow molecules to pass through the channel. In view of crystallography, zeolite and AlPO.sub.4 are fully crystalline substances, in which the atoms and channels are arranged in complete regularity. These fully crystalline molecular sieves are obtained naturally or synthesized through hydrothermal reactions. The number of fully crystalline molecular sieves obtained or synthesized thus far amount to several hundred species. They play an important role as catalysts or supports in modern chemical industries by virtue of their characteristics including selective adsorption, acidity and ion exchangeability. Examples of the current catalyst processes utilizing the characteristics of zeolite include the petroleum cracking reaction using ZSM-5 and the aromatic conversion reaction of paraffin using KL-zeolite impregnated with platinum. A significant problem of the fully crystalline molecular sieve is that it cannot be used in reactions of molecules larger than 1.3 nm in size.
Recently, a group of researchers at Mobil Co. reported a series of mesoporous molecular sieves, named the M41S series, including MCM-41 and MCM-48. See U.S. Pat. Nos. 5,057,296 and 5,102,643. These molecular sieves show a structure in which mesopores uniform in size are arranged regularly. Existing molecular sieves have been produced by using inorganic or organic cations as templates, whereas those mesoporous molecular sieves are synthesized through a liquid crystal template pathway by using surfactants as templates. These mesoporous molecular sieves have the advantage that their pore sizes can be adjusted in a range of 1.6 to 10 nm by controlling the kinds of surfactants or synthetic conditions employed during the production process.
Stuky, a professor at the University of California, U.S.A. and his colleagues reported mesoporous molecular sieves, designated as SBA-1, 2, 3, in Science, 268, 1324 (1995). The channels of the mesoporous molecular sieves are regularly arranged, while the constituent atoms show an arrangement similar to that of amorphous silica.
Mesoporous molecular sieves have regularly arranged channels larger than those of existing zeolites, thus enabling their application to adsorption, isolation or catalyst conversion reactions of relatively large molecules. Of the aforementioned mesoporous molecular sieves, the most widely researched is MCM41, which has a uniform structure exhibiting hexagonal arrangement of straight mesopores, such as honeycomb, and has a specific surface area of about 1,000 m.sup.2 g.sup.-1 as measured by ordinary BET. After being subjected to calcination to remove template materials, MCM-41 samples disclosed by early researchers undergo, although there is a little difference depending upon synthetic conditions, structure contraction ranging from 20 to 25% relative to pre-calcination. This contraction is attributed to the fact that silanol groups are condensed by the calcination.
However, the research data from the present inventors show that if the equilibrium of the silicate condensation reaction is shifted toward the product by controlling the pH of the reactants during the hydrothermal synthesis of MCM-41, the condensation of the silanol group is completed in advance, so that the weak thermal stability can be overcome. This achieves a considerable improvement in structural uniformity as disclosed in Journal of Chemical Society, Chem. common., 1995, p711. The MCM-41 thus obtained did not undergo structural decomposition even at 500.degree. C. under 1 atm inthe presence of 100% water vapor. It contracted only slightly in structure even upon heating up to 900.degree. C. under an oxygen atmosphere. A modified MCM-41 in which aluminum is substituted for silica has an acidity and ion exchangeability similar to that of existing zeolites. The modified MCM-41 can be used for various reactions requiring such properties.
Superior in thermal stability as it is, MCM-41 begins to lose its structure in water heated at 65.degree. C. or higher, which is ascribed to the hydrolysis of the silicate constituents. Twelve hours after being heated in boiling water at 100.degree. C., MCM-41 completely loses its structural characteristics. This poor hydrothermal stability serves as a serious limiting factor at temperatures 60.degree. C. or higher. Three examples include: (1) the case of using a titanium-substituted molecular sieve in a partial oxidation reaction; (2) the case of a catalyst conversion reaction under a hydrothermal condition in which hydrogen peroxide is used as an oxidizing agent; or (3) the case of impregnating in the molecular sieve a transition metal, such as platinum, as a catalytically active ingredient. Further, when MCM-41 is subjected to calcination to remove templates, it is decomposed to form coke which is, in turn, likely to close the mesopores because MCM-41 has straight channels. Even when catalytically active ingredients, such as platinum or palladium, are impregnated, the passages are readily closed, which prevents molecules from diffusing. Thus, the metal molecules present on the inner side of the closed passages cannot be in contact with reactants and only those present at the opposite ends show catalytic activity. Therefore, active research and development efforts have been and continue to be directed to an improvement in the hydrothermal stability of mesoporous molecular sieves and in overcoming the molecular diffusion problem attributable to closed channels.